1. Field of Invention
This invention relates to an image forming apparatus for forming images by the electrophotography method by using electrically charged toner. In particular, this invention relates to a unit for detecting the amount of remaining toner within a toner cartridge.
2. Description of Related Art
Most of conventional image forming apparatuses have a detachable toner cartridge for accommodating toner used for development, so that the toner cartridge must be unloaded to replace toner the remaining amount of which has become low after use for a certain period of time. Therefore, in order that users can determine a toner replacement time, a toner remaining detection unit is often provided to display the amount of remaining toner. Such a display method is not required to display an accurate amount of remaining toner. Generally, users are fully satisfied if they can determine whether the toner is in a full state in which a sufficient amount of toner remains, it is empty to such an extent that it must be replaced, or it is in low state in which it must be replaced in near future.
As a toner remaining detection unit of this sort, for example, there is known a method by which the time is measured from when toner is removed from a slot provided in a lower portion of a toner cartridge and to when it fills the slot again in such a way to periodically sweep the slot using a wiper and detecting the existence of light passage through the slot. Since the time is short when a large amount of toner remains, the amount of remaining toner can be detected by comparison with a predetermined reference time.
If a small number of levels, such as the three levels described above, is provided to detect the amount of remaining toner, the detected state is not stationary but variable during transition among the detection levels. For example, once a transition has been made to a low state or an empty state, it is normal to perform control so as not to return the indication of the remaining amount to a previous state.
However, in the above mentioned toner remaining detection method, there is a problem when toner fluidity becomes bad immediately after power on, toner replacement, or the like. That is, in the above mentioned example, when the slot is swept by a wiper, if toner fluidity is low, it takes a long time for toner to fill the slot again, and an incorrect state may be displayed. One example is that the low state is displayed after toner replacement although the toner remains full. In this case, when toner fluidity increases in the course of toner use, a full state indication is obtained. However, as described above, since the existing display state is not returned to the previous display state, it is difficult to correctly display the amount of remaining toner.
To avoid such an incorrect determination, for example, when the result of determining the amount of remaining toner changes from the low state to the full state, since the result of determining the amount of remaining toner depends on the change in toner fluidity, the empty state display may suddenly appear before the low state is displayed for a sufficient period of time. This poses a problem to users in terms of operability.